I have a potential rear main seal leak,don't know exactly if it is the rear main that's leaking so I was thinking first before I spend a fortune on repairs if I was putting a can of engine sealer first or thicker oil grade, (right now it's got castrol 5w30),could it be the source of my leak, is Castrol any good, anybody tried the engine oil leak sealers for the Mini, can somebody help me out, thanks guys !

 

Sealers, are usually just a chemical that makes shrunken seals and gaskets swell...it is temporary in that the gasket/seal will shink, sometimes to a smaller size when it is not used....so it becomes like a drug addiction for a car. Some "high mile" older car oil has a bit in it....so the stuff is a mixed bag...if a seal is physically bad, cracked or torn, this stuff will not work. The additional negarive is ALL gaskets in the motor is getting bathed in this stuff...not just the one you ned fixed.
Mini's have some common leaks....safe to say those common ones have been looked at? Syenthic oil (100% needed imo) does flow, and makes leaks look very bad....it can be blown around too...

 

Not only that, but when standard seals that aren't leaking absorbs this, they will stretch and become weak when the product is no longer used, and can distort baskets, making them leak, or need replacement.